The answer is found in evolution. Through evolution nature created the Big Cats. Conventionally, these are referred to as cats who have the ability to roar, namely the leopard, lion, tiger and jaguar.
Evolution has not provided us with wild dogs which are as big as the Big Cats. As I understand it, the argument as to why this happened is that cats normally hunt individually by using their strength, claws and teeth and short runs to attack prey whereas dogs wear down their prey through stamina and kill in packs.
Size interferes with stamina. I believe that this is the general argument, as to why the largest dogs maintained a size much smaller than the largest cats.
There was a very large wild dog, now extinct, called the Epicyon. It was native to North America. It appears to have existed up until about 5.3 million years ago. The dog weighed between 200 and 300 pounds. It had a massive head and powerful jaws. His skull was “lion-like” in shape apparently. It is described as a “bone-crushing dog”.
As this animal is extinct it no longer competes with the big cats in terms of size and weight nowadays.
In short, the answer is to be found in evolution as described by Darwin. And the style of hunting appears to have dictated how these animals evolved. However, I will be very happy to accept better answers and I tried to keep this page straightforward in the interests of readability.